Monday, 5 October 2015

Custard

I'm tempted to put this whole blog entry in yellow, but it'd be illegible so I won't bother.

This is another home ed staple - oobleck. Once again I'll start off by describing the practicalities and then move on to what it's all about.

Oobleck is a mixture of corn starch and water often referred to as custard. However, it isn't really good quality custard in terms of food but an illustration of the weird things liquids can do sometimes. The recipe is one part of water to one part of cornstarch, thoroughly but slowly mixed. You end up with a liquid which will flow if poured, but if you hit it hard it will feel firm. If you squeeze it in your hand, it will form a ball but that ball will then gradually trickle between your fingers and drip off it. If you plunge a fist into a jug of it slowly, you can then lift the jug with your hand. It also does entertaining things on speaker cones:

If you fill a swimming pool with it and stomp your way across it, it will support your weight:

Custard, as Halfbakers call it, is one of many "non-Newtonian fluids". This needs to be explained. In the case of custard, the property is shear thickening. I should probably explain what a Newtonian fluid is first. A Newtonian fluid is one whose deformation is linear with respect to stress. In other words, the harder you splash it, the further it goes. What with the world being an imperfect place, where for example lightning doesn't strike in a straight line and trees are not brown cylinders with green spheres on top, strictly speaking most liquids are not Newtonian. For instance, pond skaters can do this:

and it's possible to float needles and even razor blades on water, meaning that there can be quite a lot of stress applied to water before it starts to behave in a Newtonian manner. Air and other gases are also fluids, and are closer to being Newtonian than water. Water is in various ways a very unusual substance, one of which is its surface tension, which is stronger than most other liquids, an exception being liquid selenium.

Other examples of very non-Newtonian fluids are quicksand, wet cement, toothpaste, ketchup, silly putty and non-drip gloss paint. From a home ed point of view, lots of these are easily available.

Ketchup is notoriously good at getting stuck in a bottle and then suddenly splurting out when you bang it too hard on the bottom while gradually flowing out under the influence of gravity, although recent redesigns of ketchup bottles have made this less problematic. This is the opposite to custard, shear-thinning, and is due to the presence of xanthan gum. That is, the stress put on ketchup makes it flow more than expected, or rather, than if it was water or even a much thicker liquid like mercury.

Non-drip gloss paint is an interesting one because it's easy to spread on a surface but won't flow down it because its viscosity (thickness) depends on time more than force.

Another kind of non-Newtonian fluid is toothpaste, which is like water as described above only more so. It can hold a peak because it requires a certain amount of force before it'll move at all, but after that point behaves quite normally. (Incidentally, toothpaste is easy to make from chalk powder or dried clay with hopefully vegetarian glycerine.)

The quicksand problem is an example of the effect of shear-thinning. Quicksand itself is about twice as dense as a human body so it should be impossible to sink entirely into it. The thing to do to escape is to move slowly. However, it can become so sticky that it would take a car to pull someone out and it also means things can happen like the tide coming in before you get out or a Boa constrictor coming up and eating you. There was a point in the 1960s when 3% of all newly released films had a scene where someone got stuck in quicksand.

Silly putty is a particularly nice example of a non-Newtonian fluid. I won't link to it because that would be advertising, but for people who haven't experienced it, it's a substance which is like modelling clay when you move it slowly but if you throw it, it will bounce, and if you drop it off a high building it will actually shatter. Left to itself, it will gradually form a pool, which however can have started off as any shape. Technically, silly putty is mainly polydimethylsiloxane, which is a polymer (chain of molecules) made of silicon, oxygen, carbon and hydrogen:

It's a fairly exciting substance because it behaves like the kind of substances which make up living things, such as latex, but is largely silicon-based, raising the as yet unanswered question of whether there's life out there in the Universe based on silicon rather than carbon. I made a video about that once which was quite popular. Anyway, the reason it behaves as it does is that its chains are very flexible, so it can hold its shape well over short periods but over long periods tends to droop under the influence of mechanical forces such as gravity. It can also be used in hair conditioner.

At very low temperatures, helium becomes a very special kind of fluid called a superfluid. This is particularly interesting because it flows uphill and leak through solid objects. It has no viscosity at all, so it can be used to produce a fountain which never stops flowing:

One thing which interests me about superfluid liquid helium is what would happen if you tried to make custard with it. That is, if you took a fine powder of frozen hydrogen and suspended it in liquid helium, what would happen?